The disease that Toxoplasma causes in humans ranges from essentially asymptomatic to debilitating or even fatal. There are likely many reasons for this difference but unambiguous evidence from animal studies and recent data from clinical studies in humans indicate that strain differences in the parasite likely play a major rol. Genetic studies of the parasite have led to the identification of a small number of genes that differ between strains and that interface with the innate immune system in dramatically different ways. The on-going goals of this work are to identify these proteins and determine the molecular basis for their strain-specific effects. In the prior funding period, it was discovered that the recruitment of host mitochondria to the parasitophorous vacuole is a strain-specific phenomenon and the gene responsible for this difference, mitochondrial-association factor 1 or MAF1, was identified. The first aim of the current proposal is to determine how MAF1 recruits mitochondria, the precise nature of the strain-specific differences in its action and the downstream consequences in terms of interactions with the host. Mitochondria are being increasingly recognized as key to innate immune functions and so these differences are likely to play out in important ways in pathogenesis. Macrophages are key players in some of the earliest immune responses and progress in the first funding period revealed a role for several polymorphic Toxoplasma genes in mediating the response of these cells. The second aim of the current proposal is to identify the genes responsible for two of these strain-specific differences and determine the molecular basis for the downstream effects that are observed. Both aims will be accomplished through a combination of molecular genetics, imaging and in vivo studies.